In an internal combustion (IC) engine having a variable turbine geometry (VTG) type turbocharger, the resistance of the turbine and energy delivered by the turbine to the compressor can be controlled by adjusting the flow area of the intake of the turbine. When using such VTG, adjustment of the flow area may be achieved by rotating the turbine vanes in a certain position or transferring a sliding wall within the turbine to a certain position. Adjustment of the VTG has a direct effect on the pressure in the inlet and exhaust manifolds. The braking power of a compression brake in an IC engine depends on the gas pressure in the inlet and exhaust manifolds so that the control of the flow area of the VTG provides the possibility to control the braking power. In particular, varying the flow area of the VTG controls the braking power.
However, during the time a compression release brake is active, the relation between e.g. the vane position of the VTG turbine and gas pressure in the inlet and exhaust manifolds is not constant. This may be due to the fact that the gas temperature in the engine does not remain constant and hardware parts expand or shrink with as a function of the temperature, which can result in alterations in leakage flow and functioning of that hardware (e.g. a change in the actual VTG position due to changes in expansion ratio of linkage arms). Furthermore, piece-to-piece variation of the turbocharger results in a spread in mass flow and actual VTG position, which ultimately will result in a spread of the gas pressures in the inlet and exhaust manifolds.
Variations in the VTG position can be particularly a problem when, the flow area of the intake of the turbine is small. In these situations the gas pressure in the exhaust manifold is very sensitive for the VTG position. A slight error in VTG position can result in a large gas pressure deviation, in the exhaust manifold.
This system behaviour makes it impossible or at least very difficult to obtain a fast, response with stable and reliable braking power output on the basis of preselected VTG positions. This effect has a profound impact on the usability of the compression release brake. In a reliability aspects it may be a risk for various engine components, which can break down in case too ouch gas pressure in the exhaust manifold and/or too much engine braking torque is generated. For manual engine braking, but also for engine braking requested by vehicle functions such as cruise control, the variance in engine brake power can cause comfort problems or even safety issues.
Therefore, when using a turbocharger equipped with VTG, it is essential to control the VTG position based on a closed loop control on the gas pressure in the inlet and/or exhaust manifolds to provide constant and reliable braking power with a fast response. The closed loop control adjusts the vane or sliding wall, position of the turbine such that for a particular set of engine parameters, maximum braking power can be achieved.
Certain turbine parameters however, such as the turbine speed, may not exceed a predetermined maximum. Therefore, at higher engine speeds, controlling the VTG flow area to small values would substantially increase the risk that the turbine speed exceeds a maximum tolerable turbine speed. This effect may substantially reduce the operating range of the engine brake. Hence, for known engine brake control systems it is not possible or it is at least very difficult to maintain maximum braking power at high motor speeds and/or at nigh altitudes.
Hence, there is a need in the art for improved method and systems for controlling engine braking of an engine comprising a variable turbine geometry turbocharger.